The present invention is generally directed to a fluid control circuit and method of operating pressure responsive equipment in which the equipment normally requires a low initial force requirement which, during its cycle of operation, increases substantially. Hydraulically operating forging presses represent one class of such equipment. That is, at the beginning of a forging stroke, the volume of material in the billet undergoing plastic strain is relatively small. Towards the end of the stroke and as the material more completely fills the forging die, the volume of material undergoing plastic strain increases greatly and, in consequence, the force requirement for operating the forge increases.
Another type of equipment are blowout preventers which are conventionally equipped with ram-type preventers with blind-shear rams. Such rams include cutting blades which are used in emergencies to sever a drill pipe. At other times, the blind-shear rams function as ordinary blind rams. In the pipe shearing operation, the rams require minimal operating force until their cutting edges contact the pipe to be cut. As the pipe begins to collapse, the force needed to move the rams inwardly increases rapidly to a maximum during the actual pipe cutting.
However, in the past the control and operating circuits for such equipment have been subject to various problems. For example, pressure responsive equipment such as forging presses and blowout preventers have typically used pressure accumulators for storing and providing the necessary operating power. However, because the accumulators discharge in a relatively rapid manner, the accumulators can supply maximum force at the time of minimum need, but only minimum force at the time of maximum need. Various features of the present invention are the provision of a dedicated secondary fluid supply reserved for supplementary use and/or a dedicated fluid supply reserved for exclusive use in a branch circuit, automatic means for applying the reserve fluid supply upon a sensed demand, independently operable means for supplying fluid pressure at a plurality of fluid levels to a pressure responsive equipment, a circuit and control means for supplying higher operating force when required, and means for preventing loss of enhanced secondary operating energy into a primary operating circuit.